1. Field of the Invention
This invention relates to an acceleration sensor, and particularly to an acceleration sensor suitable for detecting variation of speed which occurs due to collision and so on, of a vehicle.
2. Description of the Related Art
As this type of an acceleration sensor, U.S. Pat. No. 4,827,091 discloses an acceleration sensor comprising a housing of conductive material, a magnetized inertia member which is mounted in the housing so as to be freely movable in a longitudinal direction of the housing, a conductor provided on at least one end surface of the magnetized inertia member in the longitudinal direction of the housing, a pair of electrodes which are disposed at one side of the longitudinal direction of the housing and are electrically connected together through the conductor when contacted with the conductor of the magnetized inertia member, and an attractor of magnetic material which is disposed at the other end side of the longitudinal direction of the housing and magnetically attracting the magnetized inertia member.
In this acceleration sensor, the attractor attracts the inertia member, and thus the magnetized inertia member stands still at the other end side inside of the housing when no or little acceleration is applied to the acceleration sensor.
When some large acceleration is applied to the acceleration sensor, the magnetized inertia member is moved against the attraction force acting between the magnetized inertia member and the attractor. During movement of the magnetized inertia member, an induced current flows in the housing, and the magnetized inertia member receives a magnetic force which urges the magnetized inertia member in an opposite direction to the moving direction. Therefore, the magnetized inertia member is kept braked, and its moving speed is reduced.
When the acceleration is lower than a predetermined value (threshold value), the magnetized inertia member does not reach the end of the housing, and moves to a halfway position and stops there. Subsequently, the magnetized inertia member is pulled back to the other end side by the attraction force acting between the magnetized inertia member and the attractor.
On the other hand, when the acceleration is greater than the predetermined value (threshold value) for example, when a vehicle equipped with this acceleration sensor collides against an object, the magnetized inertia member reaches the one end side of the housing. The conductive layer of the tip surface of tile magnetized inertia member contacts the pair of electrodes to conduct electricity through the electrodes. A voltage is beforehand applied across the electrodes, so that current flows across the electrodes at the time when the electrodes are short-circuited. The collision of the vehicle is detected on the basis of this current.
A stopper is disposed at the opposite side to the magnetized inertia member with respect to the electrodes. When the magnetized inertia member with the acceleration greater than the above threshold value abuts against the electrodes and moves forwardly while pushing the electrodes, the magnetized inertia member finally abuts against the stopper. The magnetized inertia member keeps pushing against the stopper by the acceleration for a while, and for this period the conduction between the electrodes through the magnetized inertia member continues. As described above, the electrical conduction between the electrodes occurs for some long time, whereby the collision is electrically detected on the basis of this electrical conduction in a collision detection circuit.
However, in the conventional acceleration sensor, the magnetized inertia member is repelled by the stopper when the magnetized inertia member abuts against the stopper, and thus there occurs a case where a time for the conduction between the electrodes is shortened.
Further, in the conventional acceleration sensor, when the magnetized inertia member abuts against the stopper, the magnetized inertia member repetitively contacts with and separates from the stopper, and there frequently occurs chattering in the electrical conduction between the electrodes. That is, the magnetized inertia member abuts against the stopper and slightly repelled back. Thereafter, the magnetized inertia member is accelerated, and abuts against the stopper again and repelled back again. Subsequently, the magnetized inertial member is accelerated again and abuts against the stopper again. Such contact with (abutting against) and separation from the stopper are repeated. Such repetitive motion of the magnetized inertia member in the forward and backward directions as described above causes the electrodes to be frequently electrically interrupted, and thus the chattering is induced.